The opinion written by Justice Ginsburg, in a 5-4 vote, affirmed the three-judge panel and permits the independent redistricting commission. (This vote total is deceptive: two justices would have dismissed the case for lack of jurisdiction, effectively allowing the law to remain on the books; so, in theory, 7 justices agreed with the result in some remote way, but only 5 agreed on the merits of the constitutional claim.)

First, the case permits the legislature to have standing, distinguishing it from Raines v. Byrd, in which individual members attempted to assert standing. This, I think, portends poorly for the legislators suing in Hickenlooper v. Kerr, the Guarantee Clause case I've written about before.

Second, the opinion spends a substantial amount of time emphasizing that federal statute permits the exercise of power here. But that only addresses half the question: the other half the constitutional question, because if Congress lacked the power to authorize such an exercise of power, then the statute would fall, too.

Third, on the Elections Clause issue, the bulk of the analysis turns on a generous definition of the word "legislature," including the power to delegate authority:

To restate the key question in this case, the issue centrally debated by the parties: Absent congressional authorization, does the Elections Clause preclude the people of Arizona from creating a commission operating independently of the state legislature to establish congressional districts? The history and purpose of the Clause weigh heavily against such preclusion, as does the animating principle of our Constitution that the people themselves are the originating source of all the powers of government.

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As well in Arizona, the people may delegate their legislative authority over redistricting to an independent commission just as the representative body may choose to do.

Fourth, Chief Justice Roberts pens the principle dissent relying on several core arguments: that the word "Legislature" relating to the election of Senators is instructive; the definition of the legislature; etc.

But, he also writes about the power of delegation, with some doubt:

The majority concedes that the unelected Commission is not “the Legislature” of Arizona. The Court contends instead that the people of Arizona as a whole constitute “the Legislature” for purposes of the Elections Clause, andthat they may delegate the congressional districting authority conferred by that Clause to the Commission. Ante, at 25. The majority provides no support for the delegation part of its theory, and I am not sure whether the majority’s analysis is correct on that issue. But even giving the Court the benefit of the doubt in that regard, the Commission is still unconstitutional.

Fifth, the sides dispute the functionalist definition of "legislature." From Justice Ginsburg:

THE CHIEF JUSTICE, in dissent, features, indeed trumpets repeatedly, the pre-Seventeenth Amendment regime in which Senators were “chosen [in each State] by the Legislature thereof.” Art. I, §3; see post, at 1, 8–9, 19. If we are right, he asks, why did popular election proponents resort to the amending process instead of simply interpreting “the Legislature” to mean “the people”? Post, at 1. Smiley, as just indicated, answers that question. Article I, §3, gave state legislatures “a function different from that of lawgiver,” 285 U. S., at 365; it made each of them “an electoral body” charged to perform that function to the exclusion of other participants, ibid. So too, of the ratifying function. As we explained in Hawke, “the power to legislate in the enactment of the laws of a State is derivedf rom the people of the State.” 253 U. S., at 230. Ratification, however, “has its source in the Federal Constitution” and is not “an act of legislation within the proper sense of the word.” Id., at 229–230.

Constantly resisted by THE CHIEF JUSTICE, but well understood in opinions that speak for the Court: “[T]he meaning of the word ‘legislature,’ used several times in the Federal Constitution, differs according to the connection in which it is employed, depend[ent] upon the character of the function which that body in each instance is called upon to exercise.” Atlantic Cleaners & Dyers, Inc. v. United States, 286 U. S. 427, 434 (1932) (citing Smiley, 285U. S. 355). Thus “the Legislature” comprises the referendum and the Governor’s veto in the context of regulating congressional elections.

Chief Justice Roberts also addresses the functionalism argument of "legislature":

As a matter of ordinary language and common sense, however, a difference in function does not imply a difference in meaning. A car, for example, generally serves a transportation function. But it can also fulfill a storage function. At a tailgate party or a drive-in movie, it may play an entertainment function. In the absence of vacancies at the roadside motel, it could provide a lodging function. To a neighbor with a dead battery, it offers an electricity generation function. And yet, a person describing a“car” engaged in any of these varied functions would undoubtedly be referring to the same thing.

Sixth, the Court struggles with congressional election precedent (more on this later--I have much to say!). It examine Baldwin v. Trowbridge, as Congress examined whether to seat someone elected pursuant to the state legislature's rules or the constitutional rules. From Justice Ginsburg:

The House Elections Committee, in a divided vote, ruled that, under the Elections Clause, the Michigan Legislature had the paramount power.

As the minority report in Baldwin pointed out, however,the Supreme Court of Michigan had reached the opposite conclusion, holding, as courts generally do, that state legislation in direct conflict with the State’s constitution is void. Baldwin, H. R. Misc. Doc. No. 152, at 50. The Baldwin majority’s ruling, furthermore, appears in tension with the Election Committee’s unanimous decision in Shiel just five years earlier. (The Committee, we repeat,“ha[d] no doubt that the constitution of the State ha[d]fixed, beyond the control of the legislature, the time for holding [a congressional] election.” Shiel, H. R. Misc. Doc. No. 57, at 351.) Finally, it was perhaps not entirely accidental that the candidate the Committee declared winner in Baldwin belonged to the same political party as all but one member of the House Committee majority responsible for the decision.

And from Chief Justice Roberts:

The House Elections Committee explained that the Elections Clause conferred power on “the Legislature” of Michigan to prescribe election regulations. “But,” the Committee asked, “what is meant by ‘the legislature?’ Does it mean the legislative power of the State, whichwould include a convention authorized to prescribe fundamental law; or does it mean the legislature eo nomine, as known in the political history of the country?” Id., at 47. The Committee decided, and the full House agreed,that “the Legislature” in the Elections Clause was the “legislature eo nomine”—the legislature by that name, a representative body.

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The report cites a Michigan Supreme Court precedent that allegedly reached a contrary result, but that case turned entirely on state constitutional questions arising from a state election—not federal constitutional questions arising from a federal election. . . . In any event, to the degree that the two precedents are inconsistent, the later decision in Baldwin should govern

[n.3] The majority’s suggestion that Baldwin should be dismissed as an act of partisanship appears to have no basis, unless one is willing to regard as tainted every decision in favor of a candidate from the same party as a majority of the Elections Committee.

Seventh, Justice Scalia (joined by Justice Thomas) writes that this is not within the Article III power of the courts, particularly citing the shaky foundation of Coleman v. Miller as a basis for asserting authority over the case. And Justice Thomas (joined by Justice Scalia) noted that this case is not truly about deferring to state ballot initiatives, as the Court has felt comfortable striking down such regulations before.

Not many surprises in the opinions, I don't think. But, there is much to delve into in the months ahead.